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The ideas of Ptolemy were accepted in an age when standards of scientific accuracy and proof had not yet been developed. Even when Polish astronomer Nicolaus Copernicus developed his model of a Sun-centered universe, published in 1543, he based his ideas on philosophy instead of new observations. Copernicus’s theory was simpler and therefore more sound scientifically than the idea of an Earth-centered universe.

A Sun-centered universe neatly explained why Mars appears to move backward across the sky: Because Earth is closer to the Sun, Earth moves faster than Mars. When Mars is ahead of or relatively far behind Earth, Mars appears to move across Earth’s night sky in the usual west-to-east direction. As Earth overtakes Mars, Mars’s motion seems to stop, then begin an east-to-west motion that stops and reverses when Earth moves far enough away again. Copernicus’s model also explained the daily and yearly motion of the Sun and stars in Earth’s sky. Scientists were slow to accept Copernicus’s model of the universe, but followers grew in number throughout the 16th century. By the mid-17th century, most scientists in western Europe accepted the Copernican universe.

In the 16th century, Danish astronomer Tycho Brahe made the most scientific and accurate observations of the universe to that time. Brahe discovered discrepancies between astronomical predictions and the actual events, and built a set of large instruments that enabled him to record the positions of the planets and stars with unprecedented accuracy. He moved to Prague, and, after his death, his observations were taken over by German astronomer Johannes Kepler. Kepler discovered that the planets orbited around the Sun in ellipses (elongated circles) with the Sun a bit off-center at one focus. This discovery was Kepler’s first law, and he developed two more laws about how the speeds and periods of the planets changed (see Kepler’s Laws). The first two laws were published in 1609 and the third was published in 1619.

Italian astronomer Galileo made major discoveries about celestial objects in our solar system with newly-invented telescopes in the early 17th century. His discoveries helped turn cosmology into a science based on observation, rather than philosophy. These telescopes are now in the Museo della Scienza in Florence, Italy.

The Italian scientist Galileo Galilei lived and worked during the same time period as Kepler. Galileo was the first astronomer to use a telescope to observe the sky and to recognize what he saw there. He saw that the Moon had craters, that Venus went through a full set of phases like the Moon, and that Jupiter had satellites, or moons, of its own. His discoveries, published in 1610, marked the scientific end of the cosmological systems of Ptolemy and Aristotle, though it took some time for his findings to be generally accepted.


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